1. Field of the Invention
The present invention relates to a mutant strain of Aspergillus sojae with an enhanced protease activity and a method of preparing a natural taste enhancer using the same.
2. Discussion of Related Art
Aspergillus oryzae and Aspergillus sojae, which are Aspergillus sp. used when preparing soy sauce, Doenjang (traditional fermented soybean paste) and the like, produce an enzyme such as protease, amylase, glutaminase, and the like. Protease degrades protein into peptide and amino acid, and glutaminase plays an important role in flavoring soy sauce, Doenjang, and the like, by converting free glutamine into tasty glutamic acid when degrading protein.
Therefore, researches are being carried out to improve an enzyme activity of protease or glutaminase that plays an important role in flavoring. There are the methods that microorganism is mutated through UV irradiation, a mutagen treatment, and an irradiation to isolate a mutant strain with high activity (H. Sekine et al, Agric. Biol. Chem., 33, 1477 (1969), S. Nasuno et al, J. Ferment. Technol., 55, 273 (1977), S. Yamamoto et al, J. Ferment. Technol., 52, 564 (1974)), or a high activity strain, in which all of two enzymes' activities are high, is manufactured by a cell fusion technique (Shingeomi Ushijima et al, Agric. Biol. Chem., 51 (4), 1051-1057 (1987), Shingeomi Ushijima et al. Agric. Biol. Chem. 51; 2781-2786 (1987), Shingeomi Ushijima et al. Agric. Biol. Chem. 54; 1667-1676, (1990), Shigeomi Ushijima et al. Agric. Biol. Chem., 55 (1), 129-136, (1991)).
Soy sauce and Doenjang are difficult to be widely used in food because contamination of various germs by fermenting it in a solid state cannot be avoided and thus high concentration table salt should be added. Therefore, the researches for using them as seasonings are being carried out with excluding the contamination of various germs by culturing a strain in a liquid state, and then with applying the culture medium to protein without adding a table salt (JP Patent Publication Nos. 8-173085 A and 8-242810 A).
As the researches progress, the researches for producing protease or glutaminase during liquid culture of Aspergillus oryzae and Aspergillus sojae are also being carried out (S. Ueno et al, Appl, Microbiol. Biotechnol., 26, 273 (1987), JP Patent Publication No. 3-277289 A, T. Yano et al, Agric. Biol. Chem., 55 (2), 379 (1991), JP Patent Publication No. 10-210967 A).
However, most of them have a low activity level. Even though glutaminase has very high activity, glutamine should be firstly cleaved by protease activity in order to expect an effect of glutaminase which converting glutamine into glutamic acid and thereby giving a savory taste to food.